Method of increasing the fatigue life and/or reducing stress concentration cracking of coiled metal tubing

ABSTRACT

A method of increasing the fatigue life and/or reducing stress corrosion cracking of metal coiled tubing made from a continuous length of strip material by shot peening one or both sides of the strip material along substantially the entire length of the strip material prior to milling the strip material into tubing. The continuous length of strip material is made up of individual strips welded together in end to end abutting relation and the welds are finished before the strip material is shot peened. Alternatively, or in addition to shot peening at least the side of the strip material that forms the interior surface of the tubing, the exterior surface of the tubing is continuously shot peened as the tubing is wound onto a reel or spool.

FIELD OF THE INVENTION

This invention relates generally to a method of increasing the fatiguelife of coiled metal tubing by producing small indentations or dimplesin the surface of the tubing along substantially its entire lengthduring the manufacturing process to induce compressive stresses in thetubing surface that resist low cycle fatigue caused by repeated coilingand uncoiling. Also such induced compressive stresses will resist stresscorrosion cracking of the tubing which commonly occurs when the tubingis exposed to hydrogen sulfide (H₂ S) in oil and gas wells.

BACKGROUND OF THE INVENTION

The coiled tubing of the present invention is made out of a suitablemetal such as high strength low alloy carbon steel and is primarilyintended to be used in the oil and gas well servicing industry. For thisparticular application, the tubing typically has a diameter between 1 to5 inches and a length between 12,000 to 20,000 feet, and is wound onto areel or spool, the diameter of which must be restricted due totransportation requirements. For this reason the plastic limits of thetubing are exceeded during the initial coiling process, and when thetubing is deployed into a well bore to different depths. Also, in manycases, upon completion of the required work, the tubing is rewound ontothe spool and moved to another well for reuse. During such coiling anduncoiling, the tubing is subjected to low cycle fatigue which producescracks that ultimately cause the tubing to fail. Also, stress corrosioncracking of the tubing commonly occurs when the tubing is exposed tohydrogen sulfide in oil and gas wells.

Due to the nature of the coiling and uncoiling operations and the strainlimits of the tubing material, low cycle fatigue is unavoidable.However, to the extent that the development and growth of fatigue crackscan be retarded, the working life of the coiled tubing string will beproportionately extended. Likewise, the working life of the tubing willbe extended if the stress corrosion cracking of the tubing is reduced.

SUMMARY OF THE INVENTION

The present invention relates to a method of increasing the fatigue lifeof a coiled metal tubing string by retarding the development and growthof fatigue cracks in the tubing during coiling and uncoiling in order toextend the working life of the tubing string. Also, the inventionrelates to a method of reducing stress concentration cracking of suchtubing when exposed to a corrosive environment such as hydrogen sulfide.

In accordance with one aspect of the invention, small indentations ordimples are mechanically formed in the surface of the coiled tubingalong substantially its entire length to induce compressive stresses inthe metal surface that retard the development and growth of fatiguecracks in the coiled tubing during coiling and uncoiling and resiststress corrosion cracking thereby extending the working life of thecoiled tubing.

In accordance with another aspect of the invention, substantially theentire surface of the coiled tubing is subjected to a shot peeningprocess to induce compressive stresses in the tubing surface that resisttension stresses from coiling and uncoiling and reduce stress corrosioncracking.

In accordance with another aspect of the invention, the coiled tubing ismade from a continuous length of metal strip material, the surface ofwhich is shot peened along substantially its entire length before thestrip material is milled into tubing.

In accordance with another aspect of the invention, the strip materialis made up of individual metal strips welded together, and the shotpeening operation is performed on one or both sides of the stripmaterial along substantially their entire length after the individualstrips are welded together and the welds are finished.

In accordance with another aspect of the invention, after the strip weldfinishing and strip shot peening operations, strip material having alength substantially corresponding to the length of the tubing string tobe milled is wound onto a large diameter reel prior to the millingoperation.

In accordance with another aspect of the invention, substantially theentire exterior surface of the coiled tubing is shot peened after thetube milling operation to induce compressive stresses in the exteriorsurface that resist tension stresses from coiling and uncoiling andreduce stress corrosion cracking.

These and other objects, advantages, features and aspects of the presentinvention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of butseveral of the various ways in which the principles of the invention maybe employed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic perspective view of a coiled tubing string made inaccordance with the present invention wound onto a reel or spool;

FIG. 2 is an enlarged fragmentary side elevation view, partly insection, of a portion of the tubing of FIG. 1 schematically showingsmall indentations or dimples in the exterior and interior surface ofthe coiled tubing along substantially the entire length of the tubing toinduce compressive stresses in the tubing surface that resist tensionstresses from coiling and uncoiling and reduce stress concentrationcracking;

FIG. 3 is a schematic transverse section through the tubing of FIG. 2,taken generally along the plane of the line 3--3 thereof;

FIG. 4 is a schematic diagram of a continuous strip assembly line inwhich both the upper and lower surfaces of the strip material are shownbeing shot peened after the strip end joining and finishing operationsand before the strip material is wound onto a large diameter reel priorto being milled into tubing;

FIG. 5 is an enlarged schematic perspective view showing two strip endswelded together in the strip assembly line prior to finishing the stripend weld;

FIG. 6 is an enlarged schematic perspective view of a strip end weldsimilar to FIG. 5, but showing the strip end weld after the strip endweld finishing operation;

FIG. 7 is an enlarged transverse section through the strip assembly lineof FIG. 4 taken generally along the plane of the line 7--7 thereof;

FIG. 8 is a schematic diagram of a tube mill line in which continuousstrip material is milled into tubing and substantially the entireexterior surface of the tubing is shown being shot peened alongsubstantially the entire length of the tubing as the tubing is woundonto a reel or spool; and

FIG. 9 is an enlarged transverse section through the tube mill line ofFIG. 8 taken generally along the plane of the line 9--9 thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings, and initially to FIG. 1, thereis shown a coiled tubing string 1 in accordance with this invention madefrom a suitable metal strip material such as high strength low alloycarbon steel strip manufactured in a continuous length and wound onto areel or spool 2. The coiled tubing 1 is primarily intended to be used inthe oil and gas well servicing industry and typically has a diameter ofbetween 1 to 5 inches and a length between 12,000 to 20,000 feet.

The maximum diameter of the reel or spool 2 is restricted due totransportation requirements, and for this reason the elastic limits ofthe tubing are exceeded during the coiling and uncoiling process whichresults in low cycle fatigue that produces cracks on both the inner andouter surfaces of the coiled tubing in normal operations, ultimatelycausing the tubing to fail. Also, stress corrosion cracking of thetubing commonly occurs when the tubing is exposed to a corrosiveenvironment such as hydrogen sulfide in oil and gas wells. However, inaccordance with the present invention, small indentations or dimples aremechanically formed in the surface of the metal to induce compressivestresses in the metal surface. This drives the grains of steel closertogether, thus producing "prestressing" compressive stresses in themetal which resist the action of tensile stress caused by repeatedcoiling and uncoiling operations and also reduce stress corrosioncracking of the tubing to extend the working life of the tubing.

FIGS. 2 and 3 schematically show the mechanical indentations or dimples3 in the tubing surface which desirably substantially cover the entireinner and outer surface of the tubing throughout substantially itsentire length. In one form of the invention disclosed herein, thesurface of the tubing 1 is mechanically dimpled or indented by shotpeening both sides of a continuous length of strip material from whichthe tubing is subsequently milled as described hereafter.

A continuous length of strip material 4 is assembled in a strip assemblyline 5 such as schematically shown in FIG. 4 by uncoiling individualstrips 6 of sheet metal of substantially less length than the overalllength of the continuous strip material to be milled, welding the endsof the individual strips 6 together, finishing the strip end welds, andthen recoiling the continuous length of strip material 4 onto a largetake-up reel 7 of sufficient capacity to store continuous strip materialhaving a length substantially corresponding to a continuous length oftubing string 1 to be milled. During the strip assembly operation, ashear 8 (FIG. 4) is used to shear the strip ends to be joined atsupplementary angles, preferably 45° and 135°, respectively, to placethe weld 9 in the plane of maximum shear stress, i.e. 45° to theprincipal tension stress as schematically shown in FIG. 5, and when thestrip is formed into tubing 1 with the edges of the strip weldedtogether to form a longitudinal seam 10, the strip end weld 9 will runhelically around the tubing as further schematically shown in FIG. 2.

FIG. 5 schematically shows the ends of two strips 6 welded together withtheir abutting ends 13, 14 pressed together. To prevent burn out at theedges of the strip 6 during the welding operation, small tabs 16 of thesame base material and thickness as the strips are pressed up againstthe ends of the joint 17 between the two strips, and either tack weldedor clamped in place. Then the strip ends are welded together using asuitable strip end welder 18 (FIG. 4) such as a plasma arc welder withside wire feed or TIG welder, to make a high quality weld, moving fromone of the tabs 16 along the entire length of the joint 17 and onto theother tab 16.

Upon completion of the welding, the strip end weld 9 is desirablylightly ground and X-rayed to make certain the weld is of the desiredhigh quality so that the weld can be left in the finished tubing productwithout adversely affecting the fatigue life of the finished tubingproduct. If the weld 9 does not meet the criteria for a high qualityweld, the weld is cut out and remade in substantially the same mannerpreviously described.

Once it is determined that the weld is of the desired high quality, theexcess weld material is removed from the top and bottom sides of thestrip by grinding and/or planishing the weld 9 to finish smooth the weldand make the thickness of the weld closely correspond to the thicknessof the strip material. Then the weld is stress relieved, the tabs 16 areremoved and the edges of the strip weld are milled square and deburredas needed to make the width of the strip material at the weld closelycorrespond to the width elsewhere as schematically shown in FIG. 6.

Upon completion of the weld finishing operation, and before theassembled strip 4 is recoiled onto the large diameter reel 7, both thetop and bottom surfaces of the strip material are desirably shot peenedusing standard shot peening equipment such as the shot peening wheels 20schematically shown in FIGS. 4 and 7 to produce small indentations ordimples 3 in the metal surface which produce compressive stresses in themetal surface that resist tension stresses from coiling and uncoilingthe finished tubing, and reduce stress corrosion cracking, therebyextending the working life of the coiled tubing string.

The quality of the shot peening operation may be controlled inconventional manner by controlling the velocity of the shot, thehardness, size and weight of the individual shot, the angle of impact ofthe shot with the strip material, and the degree of coverage. The shotpeening equipment used must either be adjustable for full width coverageof the strip material passing between single shot peening wheels 20above and below the strip material, or more than one shot peening wheel20 must be placed both above and below the strip material spaced bothlongitudinally and transversely from each other as schematicallydepicted in FIG. 4 to obtain full width coverage.

If the strip edges themselves are deformed during the shot peeningoperation, a strip edge conditioning station 21 may be needed at thestart of the tube mill line 22 as schematically shown in FIG. 8 torecondition the edges of the strip material to make them straight andsquare after the strip material is paid out from the large diametertake-up reel 7 and passed through the tube mill 23 as schematicallyshown in FIG. 8.

After the strip material 4 is milled into coiled tubing 1, the tubing isheat treated and both air and water cooled by passing the tubing througha suitable heat treating station 24 and air and water cooling stations25 and 26. Then the tubing is wound onto a reel or spool 2.

Prior to winding the tubing 1 onto the reel 2, the entire exteriorsurface of the tubing 1 may be continuously shot peened by passing thetubing between a plurality of shot peening wheels 27 appropriatelyspaced and positioned in the tube mill line around the periphery of thetubing between the cooling station 26 and reel 2 as schematically shownin FIGS. 8 and 9. In this embodiment, four such shot peening wheels 27are positioned 90° apart in axially spaced relation around the peripheryof the tubing.

When the exterior surface of the tubing is shot peened as schematicallyshown in FIGS. 8 and 9, the bottom surface of the strip material 4 whichbecomes the exterior surface of the tubing 1 need not be shot peenedafter the strip weld finishing operation. In fact, the strip shotpeening operation schematically illustrated in FIGS. 4 and 7 could beeliminated altogether, but that would have the disadvantage that onlythe exterior surface of the finished tubing would be shot peened,leaving the interior surface unpeened.

Although the invention has been shown and described with respect tocertain preferred embodiments, it is obvious that equivalent alterationsand modifications will occur to others skilled in the art upon readingand understanding the specification. The present invention includes allsuch equivalent alterations and modifications, and is limited only bythe scope of the claims.

What is claimed is:
 1. A method of making a continuous length ofun-coated coiled metal tubing having increased fatigue life and/orreduced stress concentration cracking for use in the oil and gas wellservice industry comprising the steps of joining the ends of a pluralityof individual metal strips to form a continuous length of strip materialhaving a length substantially corresponding to a desired continuouslength of tubing to be milled therefrom, shot peening one side of thecontinuous length of strip material along substantially its entirelength to induce compressive stresses in such one side, continuouslymilling the continuous length of strip material into a continuous lengthof tubing with the one side of the strip material that was previouslyshot peened forming the interior surface of the tubing, continuouslyshot peening substantially the entire exterior surface of the tubingafter the tube milling step to induce compressive stresses in suchexterior surface, and thereafter coiling the continuous length of tubingonto a spool.
 2. The method of claim 1 wherein the continuous length ofstrip material is built up on a storage reel and subsequentlytransported to a tube mill where the continuous length of strip materialis unwound from the storage reel and continuously milled into tubing. 3.The method of claim 2 wherein the individual metal strips that comprisethe continuous length of strip material are welded together in end toend abutting relation and the strip end welds between the individualstrips are finished before the continuous length of strip material isshot peened on such one side and built up on the storage reel.
 4. Amethod of making a continuous length of un-coated coiled metal tubinghaving increased fatigue life and/or reduced stress corrosion crackingfor use in the oil and gas well service industry comprising the steps ofmaking up a continuous length of strip material having a lengthsubstantially corresponding to a desired continuous length of coiledmetal tubing to be milled from the strip material, the continuous lengthof strip material being formed by joining the ends of a plurality ofindividual metal strips each having one side and an other side oppositethe one side, shot peening the one side of the continuous length ofstrip material along substantially its entire length to inducecompressive stresses in such one side, coiling the continuous length ofstrip material onto a storage reel, and subsequently uncoiling thecontinuous length of strip material from the storage reel andcontinuously milling the continuous length of strip material into acontinuous length of tubing with the one side of the strip material thatwas previously shot peened forming the interior surface of the tubing,and coiling the continuous length of tubing onto a spool.
 5. The methodof claim 4 further comprising the step of shot peening the other side ofthe continuous length of strip material along substantially its entirelength to induce compressive stresses in such other side before coilingthe continuous length of strip material onto the storage reel, suchother side forming the exterior surface of the tubing during continuousmilling of the continuous length of strip material into the continuouslength of tubing.
 6. The method of claim 4 further comprising the stepof continuously shot peening substantially the entire exterior surfaceof the tubing after the continuous milling operation and before thecontinuous length of tubing is coiled onto the spool to inducecompressive stresses in such exterior surface before the continuouslength of tubing is coiled onto the spool.
 7. The method of claim 4further comprising the step of finishing the strip end welds between theindividual metal strips before such one side of the continuous length ofstrip material is shot peened.
 8. The method of claim 4 wherein bothsides of the continuous length of strip material are shot peened alongsubstantially the entire length of the continuous length of stripmaterial after the ends of the individual metal strips are joinedtogether and before the continuous length of strip material is coiledonto the storage reel.
 9. The method of claim 8 wherein the ends of theindividual metal strips are joined together by welding the strips in endto end abutting relation, and the strip end welds between the individualmetal strips are finished before the sides of the continuous length ofstrip material are shot peened.
 10. The method of claim 4 furthercomprising the step of conditioning the edges of the continuous lengthof strip material as the strip material is unwound from the storage reeland before the strip material is milled into tubing.
 11. The method ofclaim 4 wherein at least approximately 12,000 feet of the continuouslength of strip material is coiled onto the storage reel andsubsequently continuously milled into tubing having substantially thesame length as the continuous length of strip material.
 12. The methodof claim 11 wherein between approximately 12,000 to 20,000 feet of thecontinuous length of strip material is coiled onto the storage reel andsubsequently continuously milled into tubing having substantially thesame length as the continuous length of strip material.
 13. The methodof claim 11 wherein the tubing that is milled from the continuous lengthof strip material has a diameter of at least approximately one inch. 14.The method of claim 13 wherein the tubing that is milled from thecontinuous length of strip material has a diameter of betweenapproximately one to five inches.